I am trying to replace a on/off push-button with a photocell. With the push button my device works as follow; when the button is pressed the device is ON and when the button is released the device is OFF. To replace this button with a photocell I need to have the following specs. When the light is on the device is OFF when the light the off the device is ON. I tired a photocells with different resistances such as 15K-30K --- 1M-1.5M but no of them worked. When I connect the photocell it immediately shots the circuit. I think the sepcs need to be 0K to 1M+. But I was unable to find such a photocell.

If some electronic engineer can tell me what can I do to solve the problem using the photocells I already have, that would be great. Or maybe you know where I can find a photocell with 0K it would be even better.

I am sure you might have some questions so please ask and I will give you all of the information that will help to solve my problem.

WOW guys, thank you for all of this. I am really happy to have found you. You have restored my hope for this thing. I am not in a way an expert with electronics so please pardon me if I am not grasping this right away. But I will provide all the information you ask for. I am uploading the pics right now.

Pic 2) White and Blue wires that I have soldered to the button and extended them so I can try out the photocell system. In the pictures the wires are connected to the cell and the green light is on. Indicating that the button is "ON"

Pic 3) Back of the board. On the button right corner you can see where I have connected the blue and white wires.

Pic 4) The front.

Ignore the extended black wire. It just the ground wire. It has no effect on anything. I was trying somethings with it and I have just reconnected it how it was. The other black wire with the connector on the end is to charge the car.

Ok, from what I can tell, the switch shorts the input right to the ground plane, acn not see if there are any pullup resistors for the input, but that doesn't mean the controller IC does not have them built in....

So, my first assumption about the open collector transistor circuit would actually do, what you will need is a couple of resistors and a 2N3904 Transistor...

Do you have a multimeter handy? If you do measure the resistance of the CDS photocells you have on hand and see what the resistances are when fully covered and exposed to light...

Or do you know the values of the CDS photocells you have on hand (Such as the value when completely saturated with light, and when completely covered and in the dark)?

It is unclear whether you have the leads in the right sockets, because the leads are covering up the text below them.

To measure the photocell, you need to attach the probes to the photocell (for basic components, I find wrapping the wires round the leads works well as a temporary solution), set your multimeter to measure ohms on the 2M range, then record the numbers with it covered and uncovered. If the value is too small when uncovered, try ranging the multimeter down until you use all the digits on the display, remember to tell us at what range you took the measurements.

I suggest you use one of the transistor circuits suggested. You *might* be able to get away with the light sensors used in some night lights which to my knowledge are just wired in series with the light, these have to have low on resistance and they might work, but they could be difficult to find. (It might even be easier to take a night light apart and see if you can salvage the LDR.)

Before I measure the resistance on the sensors what is the reading that will work for me, or in any outcome I will have to build a some kind of circuit? In addition I can get a specific photocell from an electronics parts store in my city. That is where I go all of mine so far. Apart from the ones that I pulled from the night lights and a bunch from a garden lights. And if I have to build a circuit what parts will I need for that circuit and in what order do I put them together? And for the list of parts could you also tell me the specs for the parts. Because when I go to the store and tell them I want a resistor they ask me all bunch of specs that I want that resistor in and I have no idea what they are talking about. If you will post a diagram that I will have to use to build the circuit could you also please label it so I would understand. Because looking at the diagram that was posted earlier in this thread I realize that I don't understand it at all. I would need more simple labeling such as: "resistor goes here" and "photocell goes here" and "you connect your button here", kind of labeling. Because I am not really familiar with all the symbols that are used in the diagram.

Thank you for sticking out with me so far. I realize that I must be hard to deal with but I really appreciate this. I know how you must feel as I myself is a very knowledgeable auto-tuner and I spend a lot of time explaining performance to people who think that if the tire goes flat, the air must be put only at the button part of the tire.

Here is the store that I use to buy my parts. It might help to see what they have. Or if you know a place online that will have better parts for this job I will use that. But it would be easier to get it from my town. http://www.gervaiselectronics.ca/

In the circuit, R1 and LDR1 condition the input signal, and R2 and Q1 act to invert it.

Note that since this provides a signal to your device you will probably need to use the same power supply as the device, unless you connect the grounds and be sure you're using the same voltages; but it's easier to use the same supply.

It does not matter when the switch will be on or off. Because the light that will trip the LDR is modified by my computer. So I can adjust what the light does from the computer. The purpose for this whole thing is to be able to control the RC car with my computer. I will replace all of the switches with the LDRs so I can control the whole car. If this thing works I will post a video of this in action. But there is more to this whole project. And this car is just a beta tester. Its just a micro car. Once this works I will use the same method to hook up more substantial RC car and put a wireless cam on it so I can drive it around without even being in the same building.

THANK YOU< THANK YOU> THANK YOU> I will try this and tell you how this turns out.

If you are controlling this with your computer... why not just wire up one of the serial or parallel ports? If you don't have a serial or parallel port, you can get a converter for USB to whatever you need.